Monolithic integrated circuits, such as MMICs (Monolithic Microwave Integrated Circuits), are devices that effectively utilize the electron transport characteristics of compound semiconductor materials to create relatively high-speed FETs (Field Effect Transistors). Accordingly, application of such MMICs to microwave or millimeter wave ICs for satellites, cellular phones and other mobile communications devices has been promoted, and the demand for MMICs has increased along with the proliferation of such mobile communication devices in recent years.
A typical MMIC comprises active and passive elements, such as FETs, resistors, MIM (Metal Insulator Metal) capacitors, interdigital capacitors, spiral inductors, thin-film resistors, and various wiring layers for connecting these elements.
In a standard monolithic microwave IC, one problem is the effective fabrication of components to achieve relative compactness to reduce circuit size, thereby increasing the yield and decreasing manufacturing costs for a given fabrication process while simultaneously maintaining the desired operating characteristics of the individual components. This problem is particularly manifest in the substantial efforts required to achieve a compact capacitor with high capacitance.
During standard MMIC design, the typical circuit element of choice for circuits requiring high capacitance is the MIM capacitor. The MIM capacitor is selected for its ability to provide relatively effective capacitance values in a relatively small amount of space. Unfortunately, the standard MIM capacitor cannot always be reliably manufactured with capacitance values lower than approximately 2 pf.
While attempts to fabricate MIM capacitors with sub-2 pf capacitance values have occasionally been successful, the manufacturing yield (defined as the unacceptable variation in capacitance value) of these circuits is generally lower than desired for most mass production requirements. The low yield of MIM capacitors with sub-2 pf capacitance is caused by the tolerances relative to the dielectric thickness between the two plates of the capacitor. This problem is particularly accentuated on small capacitors due to a relatively larger percentage change in capacitor plate area, which results from typical photo-lithography process variations when printing the metal plates that form the capacitor plate area and the dielectric layers that separate the capacitor plates.
In those situations where high manufacturing yields and lower capacitance are desired, the designer will usually rely on a standard interdigital style capacitor. The typical interdigital capacitor allows for the realization of small capacitance values with less sensitivity to process variation and is, therefore, suitable for circuit designs where high yields are important. However, one potential drawback of the interdigital capacitor is that it is measurably larger in area than a traditional MIM capacitor. This means that while an interdigital capacitor produces a higher yield for smaller capacitance value capacitors, it provides relatively low capacitance when the overall capacitor-size-to-capacitance values are considered. Accordingly, circuit designers are often forced to compromise the size of an IC in order to ensure reliable circuits with a manufacturing yield that will be economically viable. This problem is only exacerbated by the increasing demand for high frequency communication circuits.
In view of the foregoing, it should be appreciated that it would be desirable to increase the yield rate of MIM capacitors with smaller capacitance values. In addition, it would be desirable to provide new methods and techniques for fabricating capacitors with smaller capacitance ratings in a smaller area without requiring the addition of new and costly processing steps. Furthermore, additional desirable features will become apparent to those skilled in the art from the drawings, foregoing background of the invention, following detailed description of the drawings, appended claims, and abstract of the invention.